Automotive radar

ABSTRACT

An automotive radar having a high level of azimuth accuracy and a broad detectable range by virtue of preventing leaks of unnecessary waves to receiving antennas is to be provided. A transmitting array antenna  1  and receiving array antennas  2   a   , 2   b  are arranged on an antenna plate (grounding conductor plate)  3  which serves to ground the antennas, and radio wave absorbers  4  are arranged on two sides of the edges  15  of the antenna plate  3 . The radio wave absorbers  4  may be configured of, for instance, a sponge material containing radio wave absorbing grains.

FIELD OF THE INVENTION

The present invention relates to an automotive radar to be mounted on amobile object, such as a wheeled vehicle, to detect the azimuth of anobstacle, its distance from the mobile object and relative speed.

BACKGROUND OF THE INVENTION

Automotive radars using millimeter waves are attracting interest asbeing most suitable for use in the prevention of car collisions and infollowing another vehicle as they are less susceptible to theinterference of rain, fog, snow and other weather factors, dust andnoise than ultrasonic radars and laser radars. Millimeter wave radarscurrently available in the market are mainly intended for use alongexpressways, and their detectable range and reach are approximately 16degrees in azimuth angle and about 150 m in distance, respectively.

Automotive radars are further expected to be applied in the near futureto a more complex road environment, such as ordinary roads, and thiswould inevitably necessitate a broader detectable range of 80 degrees ormore in azimuth angle.

Techniques available to meet this need include a method widening areceiving angle of antenna on the basis of use of a plurality ofreceiving antennas and a monopulse system of detecting the azimuth ofthe obstacle from the amplitude difference or the phase difference amongthe signals received by the receiving antennas. For instance, non-patentreference 1: 2001 General Conference of the Institute of Electronics,Information and Communication Engineers, Technical Papers (EngineeringScience), Paper No. A-17-10, p. 391 discloses a technique to broaden thedetectable range by using a monopulse system for azimuth detection andkeeping the number of antenna elements intentionally small. Besidesthat, for instance, non-patent reference 2: MWE 2001 (MicrowaveWorkshops and Exhibition) held in Yokohama in December, 2001, WorkshopTechnical Program, Paper No. WS5-1 contains a description of anautomotive radar transmitter/receiver apparatus embodied in such amethod.

FIG. 11 shows the circuit configuration of a radar transmitter/receiverapparatus described in the non-patent reference 2. For a wide-angletransmitting antenna 1 and receiving antennas 2 a and 2 b,microstrip-patch planar antennas based on microstrip lines are used. Amillimeter wave signal outputted by an oscillator 7 is supplied to thetransmitting antenna 1 via a power amplifier 6. The signal transmittedfrom the transmitting antenna 1 and reflected by an obstacle is receivedby the receiving antennas 2 a and 2 b, and supplied to a hybrid circuit(HYB) 5. The hybrid circuit 5 generates a sum signal Σ and adifferential signal Δ. The sum signal Σ and the differential signal Δare respectively processed by receiver circuits 20 a and 20 b, eachmainly consisting of a mixer to detect the azimuth and other factors ofthe object of detection.

SUMMARY OF THE INVENTION

As described above, in the monopulse system, the detecting range isbroadened by widening the angle of the antenna beam. However, a widenangle of the antenna beam inevitably brings an increase in unnecessarysidelobes (subbeams of weaker intensity radiating in differentdirections from the main beam) The sidelobes cause leaks of unnecessarywaves from the transmitting antenna to the receiving antennas, whichwould bring a deterioration in azimuth accuracy and an increase in wrongdetections.

The unnecessary waves leak via two routes. On one route, reflected wavesreturning after being Doppler-shifted by an obstacle are diffracted atan edge of a plate (conductor plate) which fixes the antennas and servesto ground the antennas are received by the receiving antennas. On theother route, the reflected waves are received by a transmitting antenna,and part of them are reflected due to mismatching within thetransmitting antenna to be radiated again to leak to the receivingantennas.

In the monopulse system, because the azimuth of the obstacle isdetermined according to the amplitude and phase of receive signals, theproblem noted above leads to significant deterioration of the azimuthaccuracy and wrong detection.

An object of the present invention is to obviate the problem noted aboveand to provide a low-cost, compact and light-weight automotive radarthat excels in azimuth accuracy over a wide detectable range.

The problem the present invention addresses can be effectively solved byproviding a diffracted wave prevention structure at edges, at leastpartly, of the top face of a grounding conductor plate which groundsantennas in an automotive radar to be mounted on a mobile object, suchas a wheeled vehicle, to detect the azimuth of an obstacle, its distancefrom the mobile object and relative speed. The diffracted waveprevention structure may comprise, for instance, a radio wave absorbercovering the top face edges at least in part and arranged on the topface of the grounding conductor plate, because the use of such means canprevent diffracted waves from being generated at the edges of thegrounding conductor plate, and unnecessary waves can be therebyprevented from leaking to the receiving antennas, accordingly to achievea high level of azimuth accuracy.

It is preferable to provide a radio wave absorber between thetransmitting antenna and the receiving antennas. This arrangement canprevent leaks from the transmitting antenna to the receiving antennas,resulting in a high level of azimuth accuracy.

Further, by configuring the radio wave absorber of a sponge materialcontaining radio wave absorbing grains, the automotive radar can beeasily produced in light weight and at low cost.

Also, by fitting in advance the radio wave absorber to a radome (aprotective cover for the radar) covering the front of the antennas, thenumber of assembling processes can be reduced.

Further by so selecting the height of the radio wave absorber from thetop face of the antennas as to prevent radio waves from radiating, orarriving from, outside the range of obstacle detecting angles,diffracted waves can be prevented from leaking from the grounding edgeor leaks from the transmitting antenna to the receiving antennas can beprevented, resulting in a further contribution to azimuth accuracyimprovement.

Also, by having the transmitting antenna protrude farther ahead than thereceiving antennas, leaks from the transmitting antenna to the receivingantennas can be prevented, resulting in a further enhanced level ofazimuth accuracy.

These and other objects and many of the attendant advantages of theinvention will be readily appreciated as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an automotive radar, which is a firstpreferred embodiment of the present invention.

FIG. 2 shows a front view of the first preferred embodiment of theinvention.

FIG. 3 shows a front view of a second preferred embodiment of theinvention.

FIG. 4 shows a front view of a third preferred embodiment of theinvention.

FIG. 5 is a diagram for explaining the optimal positions and size of theradio wave absorbers in the preferred embodiments of the invention.

FIG. 6 is a diagram for describing the method of mounting the radio waveabsorbers in the first through third preferred embodiments of theinvention.

FIG. 7 shows a sectional view of a fourth preferred embodiment of theinvention.

FIG. 8 shows a sectional view of a fifth preferred embodiment of theinvention.

FIG. 9 shows a sectional view of a sixth preferred embodiment of theinvention.

FIG. 10 shows a sectional view of a seventh preferred embodiment of theinvention.

FIG. 11 is a block diagram for describing the transmitter/receiverapparatus of an automotive radar according to the prior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An automotive radar according to the present invention will be describedin further detail below with reference to a number of preferredembodiments of the invention illustrated in accompanying drawings. Thesame reference signs in FIG. 1 through FIG. 11 denote either the same orsimilar elements, respectively.

FIG. 1 shows an automotive radar, which is a first preferred embodimentof the invention. In this embodiment, a transmit signal is transmittedfrom a radar transmitter/receiver apparatus 10 via a transmitting arrayantenna 1, and signals reflected by an obstacle are received by areceiving array antenna 2 a and an receiving array antenna 2 b. The tworeceive signals from the array antennas 2 a and 2 b are sent to theradar transmitter/receiver apparatus 10 provided with a hybrid circuit 5for generating a sum signal and a differential signal. The effectiveareas of the antenna 1 and the array antennas 2 a and 2 b are shaped innarrow and long strips, and serve to broaden the main beam in widthwisedirection.

In the radar transmitter/receiver apparatus 10, a millimeter wave signalgenerated by the oscillator 7 is supplied to the transmitting arrayantenna 1 via a power amplifier 6. On the other hand, the hybrid circuit5, into which the two receive signals from the array antennas 2 a and 2b are inputted, generates a sum signal Σ and a differential signal Δ.These signals are supplied to mixers 8 a and 8 b, respectively, to bemixed with the output signal of the oscillator 7 and converted intointermediate frequency signals, which are inputted into a signalprocessing circuit 9. The signal processing circuit 9 detects theazimuth of the object of detection by using signals resulting from thefrequency conversion of the sum signal Σ and the differential signal Δ,and detects the speed, position and other factors of the object ofdetection by using the sum signal Σ. The result of these detections areconverted, as required, into signals suitable for an output device, suchas a display 11, and supplied to the output device.

This embodiment has an antenna plate (antenna conductor plate) 3 overwhich the transmitting array antenna 1 and the receiving array antennas2 a and 2 b are arranged and which serves to ground the antennas, andradio wave absorbers 4 are arranged on two sides of edges 15 of theantenna plate 3.

FIG. 2 shows a top view of the antennas used in the first embodiment ofthe invention. A plurality of patch elements 12 and feeding lines 13 areconfigured over a dielectric substrate to constitute the transmittingarray antenna 1 and the receiving array antennas 2 a and 2 b. Each ofthese array antennas is arranged on the antenna plate 3, and the radiowave absorbers 4 are arranged on the two sides of the edges 15 of theantenna plate 3.

In case of the absence of the radio wave absorbers 4, the reflected wavewhich has been Doppler-shifted by the obstacle and returned, would bediffracted by the edges 15 of the antenna plate 3 and become scatteringunnecessary waves, part of which would come incident on the receivingarray antennas 2 a and 2 b, but in this embodiment of the invention thereflected waves directed towards the edges 15 are absorbed by the radiowave absorbers 4 and thereby prevented from reaching the edges 15.Therefore, the generation of unnecessary waves by diffraction isprevented, and so the problem of unnecessary waves leaking to theantennas 2 a and 2 b is solved. Even if there is any remaining portionof the reflected waves not completely attenuated by the radio waveabsorbers 4, any unnecessary waves generated by the diffraction of thatportion of the reflected wave by the edges 15 will be further absorbedby the same radio wave absorbers 4, and do not reach the antennas 2 aand 2 b. As a result, wrong detection can be prevented, and a high levelof azimuth accuracy can be achieved.

FIG. 3 shows an automotive radar, which is a second preferred embodimentof the invention. In this embodiment, a plurality of patch elements 12and feeding lines 13 are formed over a dielectric substrate toconstitute a transmitting array antenna 1 and receiving array antennas 2a and 2 b. These array antennas are arranged on an antenna plate 3, anda radio wave absorber 4 is disposed around the edges of the antennaplate 3.

In this embodiment of the invention, the returning reflected waveDoppler-shifted by the obstacle is absorbed by the radio wave absorber 4on its way to the edges of the antenna plate 3. The generation ofunnecessary waves is thereby prevented, and so the problem ofunnecessary waves being received by the receiving antennas 2 a and 2 bis solved.

Since the whole edges of the antenna plate 3 are covered by the radiowave absorber 4 in this embodiment, the absorption can be effectiveagainst diffracted waves not only in the lateral but also in thelongitudinal direction of the drawing and, accordingly, wrong detectioncan be prevented, resulting in a high level of azimuth accuracy.

FIG. 4 shows an automotive radar, which is a third preferred embodimentof the invention. In this embodiment, a plurality of patch elements 12and feeding lines 13 are formed over a dielectric substrate toconstitute a transmitting array antenna 1 and receiving array antennas 2a and 2 b. While these array antennas are arranged on the antenna plate3, in this embodiment particularly, radio wave absorbers 4 are arrangedbetween the transmitting/receiving antennas in addition to around theedges of the antenna plate 3.

In this embodiment of the invention, by arranging the radio waveabsorbers 4 in this way, not only is the returning reflected waveDoppler-shifted by the obstacle prevented from being diffracted by theedges of the antenna plate 3 but also can unnecessary waves directedtowards the receiving antennas 2 a and 2 b resulting from reradiationfrom the transmitting antenna 1 be absorbed and intercepted. Thereflected wave from the obstacle comes incident on not only thereceiving antennas 2 a and 2 b but also the transmitting antenna 1. Thereflected wave coming incident on the transmitting antenna 1 will bereflected, if there is any impedance mismatching within the transmittingantenna 1, and reradiated from the transmitting antenna 1. Thisreradiated component becomes unnecessary waves, part of which would comeincident on the receiving array antennas 2 a and 2 b if there were noradio wave absorbers 4.

In this embodiment of the invention, as in the second embodiment, thewhole edges of the antenna plate 3 are covered by the radio waveabsorbers, and accordingly the absorption can be effective againstdiffracted waves not only in the lateral but also longitudinal directionof the drawing, with the result that wrong detection can be preventedand a high level of azimuth accuracy can be achieved.

Hereupon, the optimal positions and size of the radio wave absorbers 4relative to the transmitting antenna 1 will be explained with referenceto FIG. 5. The transmitting array antenna 1 and the radio wave absorbers4 are arranged on the antenna plate 3, and a main beam mb having aradiation angle 2θ required for achieving a desired azimuth detectingperformance is supplied from the transmitting array antenna 1. Then,assuming that the antenna 1 is thin enough, the optimal values of theheight H of the radio wave absorbers 4 from the top face of the antenna1 and of the distance D of each of them from the center of the antennaare given by Equation (1):tan θ=D/H  (1)By so selecting D and H as to satisfy this equation, radio wavesradiated outside the range of obstacle detecting angles can beintercepted, resulting in high azimuth accuracy. For the receiving arrayantennas 2 a and 2 b as well, the optimal positions and size of theradio wave absorbers 4 can be selected so as to intercept radio wavescoming incident outside the range of obstacle detecting angles. Thus,when radio waves are reaching the receiving antennas 2 a and 2 b fromthe range of an incident angle 2θ required for acquiring desired azimuthdetecting performance, the optimal values of the height H of the radiowave absorbers 4 from the top faces of the antennas 2 a and 2 b and ofthe distance D of each of them from the center of the pertinent antennaare given by Equation (1) stated above.

It goes without saying that, while the optimal positions and size of theradio wave absorbers 4 have been explained with reference to the thirdembodiment of the invention, they can be applied to the first and secondembodiments as well.

The radio wave absorbers 4 for use in the first through thirdembodiments of the invention can be easily produced in light weight andat low cost by using a sponge material containing radio wave absorbinggrains, such as carbon grains.

FIG. 6 shows a typical process of mounting the radio wave absorbers 4 inthe first through third preferred embodiments of the invention. Whilethe transmitting array antenna 1 and the receiving array antennas 2 aand 2 b are arranged on the antenna plate 3 which is to ground theantennas, the radio wave absorbers 4 are fitted in advance to a radome14 covering the front of the antennas. This enables the radio waveabsorbers 4 to be arranged in appropriate positions at the same time asthe fitting of the radome 14 to the antenna plate 3, achieving asubstantial curtailment of the assembling process.

FIG. 7 shows an automotive radar, which is a fourth preferred embodimentof the present invention. In this embodiment, the edges 15 of theantenna plate 3 on which the transmitting array antenna 1 and thereceiving array antennas 2 a and 2 b are arranged and which serves toground the antennas are chamfered.

In this embodiment, since the edges 15 of the antenna plate 3 arechamfered, the returning reflected wave which is Doppler-shifted by theobstacle is hardly diffracted at the edges 15, and accordinglyunnecessary waves can be prevented from being generated. Therefore,wrong detection can be prevented and a high level of azimuth accuracycan be achieved. Incidentally, although the edges 15 shown in FIG. 7 arechamfered to have two faces, they can as well be chamfered to have threeor more faces with similar effectiveness.

FIG. 8 shows an automotive radar, which is a fifth preferred embodimentof the present invention. In this embodiment, the edges 15 of theantenna plate 3 on which the transmitting array antenna 1 and thereceiving array antennas 2 a and 2 b are arranged and which serves toground the antennas are curved.

In this embodiment, since the edges 15 of the antenna plate 3 arecurved, the returning reflected wave which is Doppler-shifted by theobstacle is hardly diffracted at the edges 15, and accordinglyunnecessary waves can be prevented from being generated. Therefore,wrong detection can be prevented and a high level of azimuth accuracycan be achieved.

FIG. 9 shows an automotive radar, which is a sixth preferred embodimentof the present invention. In this embodiment, the edges 15 of theantenna plate 3 on which the transmitting array antenna 1 and thereceiving array antennas 2 a and 2 b are arranged and which serves toground the antennas are provided with projections 16.

In this embodiment, since the edges 15 of the antenna plate 3 areprovided with the projections 16, the returning reflected wave which isDoppler-shifted by the obstacle is hardly diffracted at the edges 15,and accordingly unnecessary waves can be prevented from being generated.Therefore, wrong detection can be prevented and a high level of azimuthaccuracy can be achieved.

In addition, the effectiveness of this configuration can be particularlyenhanced by setting the height of the projections 16 to ¼ of thefrequency used.

FIG. 10 shows an automotive radar, which is a seventh preferredembodiment of the present invention. In this embodiment, the edges 15 ofthe antenna plate 3 on which the transmitting array antenna 1 and thereceiving array antennas 2 a and 2 b are arranged and which serves toground the antennas are stepped, with the transmitting antenna 1protruding farther ahead than the receiving antennas 2 a and 2 b.

As described above, while the returning reflected wave which isDoppler-shifted by the obstacle partly comes incident on thetransmitting antenna 1, if there is any impedance mismatching within theantenna 1, it will be reradiated. Since in this embodiment thetransmitting/receiving antennas are stepped, it is possible to reducethe reradiation reaching the receiving array antennas 2 a and 2 b. As aresult, wrong detection can be prevented and a high level of azimuthaccuracy can be achieved.

Apart from using the radio wave absorbers mentioned above andstructuring the edges of the antenna plate to be effective for theprevention of radio wave diffraction, it is also effective to configurethe antenna plate itself of a radio wave absorber. As the surface of theantenna plate absorbs radio waves, no diffraction of the radio wavesoccurs. Furthermore, it is also effective to coat the surface of theantenna plate with a radio wave absorber. As the surface of the antennaplate 3 absorbs radio waves, they are not diffracted. In either way, theantenna plate can be easily produced in light weight and at low cost. Itgoes without saying that such an antenna plate can be used as theantenna plate 3 in any of the embodiments of the invention describedabove. A high synergic effect can be obtained.

Although the antennas and the antenna plate 3 are supposed to beseparate components in the foregoing embodiments of the invention, theinvention is not limited to such configurations, but can be aseffectively applied to antennas in which the antenna plate and radiatingparts are integrated, for instance slotted waveguide antennas andtriplate antennas.

According to the present invention, since unnecessary waves can beprevented from leaking to receiving antennas, it is made possible torealize an automotive radar having a broad detectable range capable ofpreventing wrong detection and excelling in azimuth accuracy.

It is further understood by those skilled in the art that the foregoingdescription is preferred embodiments of the disclosed device and thatvarious changes and modifications may be made in the invention withoutdeparting from the spirit and scope thereof.

1. An automotive radar comprising: a transmitting antenna and receiving antennas arranged on the top face of a grounding conductor plate, and a radar transmitter/receiver apparatus for supplying a signal to the transmitting antenna and detecting the azimuth of an obstacle from signals of a wave reflected by the obstacle and received by the receiving antennas, wherein a diffracted wave prevention structure is provided at edges of the top face of said grounding conductor plate at least in part, said diffracted wave prevention structure being arranged to prevent a diffracted wave being generated through radiation from the transmitting antenna, reflection by the obstacle and diffraction at the edges and then being received by the receiving antenna.
 2. The automotive radar according to claim 1, wherein said diffracted wave prevention structure comprises a radio wave absorber covering said top face edges at least in part and arranged on the top face of said grounding conductor plate.
 3. The automotive radar according to claim 2, wherein a radio wave absorber is also arranged between said transmitting antenna and said receiving antennas.
 4. The automotive radar according to claim 3, wherein said radio wave absorbers are comprised of a sponge material containing radio wave absorbing grains.
 5. The automotive radar according to claim 3, further comprising a radome covering the front of said transmitting antenna and said receiving antennas.
 6. The automotive radar according to claim 5, wherein said radio wave absorbers are fitted in advance to the radome.
 7. The automotive radar according to claim 3, wherein the height of said radio wave absorbers from the top face of said transmitting antenna is so selected as to prevent radio waves from radiating outside the range of obstacle detecting angles, and their height from the top faces of said receiving antennas is so selected as to prevent radio waves from arriving from outside the range of obstacle detecting angles.
 8. The automotive radar according to claim 2, wherein said radio wave absorber is comprised of a sponge material containing radio wave absorbing grains.
 9. The automotive radar according to claim 2, further comprising a radome covering the front of said transmitting antenna and said receiving antennas.
 10. The automotive radar according to claim 9, wherein said radio wave absorber is fitted in advance to the radome.
 11. The automotive radar according to claim 2, wherein the height of said radio wave absorber from the top face of said transmitting antenna is so selected as to prevent radio waves from radiating outside the range of obstacle detecting angles, and its height from the top faces of said receiving antennas is so selected as to prevent radio waves from arriving from outside the range of obstacle detecting angles.
 12. The automotive radar according to claim 1, wherein the top face edges of said grounding conductor plate are at least in part made up of at least two faces.
 13. The automotive radar according to claim 1, wherein the top face edges of said grounding conductor plate at least in part have a curved face.
 14. The automotive radar according to claim 1, wherein the top face edges of said grounding conductor plate are provided with projections. 